1. Field of the Invention
The present invention relates to a light guide plate which is supplied with light sideways and direction-converts the supplied light inside to output an oblique emission, to a surface light source device employing the light guide plate and a direction modifying element in combination and to a liquid crystal display employing the surface light source device for illumination a liquid crystal display panel.
2. Description of the Related Art
A well-known technique employs a light guide plate having an incidence end face provided by a side end face and an emission face provided by one of two major faces (i.e. faces larger than end faces), wherein light is introduced into the light guide plate through incidence end face. A light guide plate used in such a way is applied to a surface light source device, which is employed for various uses such as back-lighting for a liquid crystal display. Surface light source devices of such a type are subject to a basic performance depending greatly on light guide plates employed therein.
A basic function of a light guide plate is to change a propagation direction (roughly in parallel with an emission face of the light guide plate) of light introduced into the light guide plate through a side end face so that the light is emitted through the emission face. As known well, a simply transparent light guide plate without any modification is capable of deflecting light a little, providing a unsatisfactory brightness. Consequently, any means for promoting emission through the emission face is required.
In general, means for promoting emission relies upon one of the followings or some of them as combined: (1) Scattering power within a light guide plate (light scattering guide plate); (2) Emission face (a major face) provided with light diffusibility; (3) Back face provided with light diffusibility; (4) Emission face provided with light-refractive unevenness; and (5) Back face provided with light-refractive unevenness.
Ways based on (1) provide uniform and highly effective emission with ease. However, the emission is directed to a preferential direction much inclined with respect to a frontal direction. Usually, the inclination is about 60 to 75 degrees to a normal with respect to the emission face. Further to this, a main beam is accompanied by a remarkable quantity of light that propagates almost along the emission face.
An direction modifying element (prism sheet) is capable of direction-modifying from the inclined direction to the frontal direction. However, the light propagating almost along the emission face is hardly direction-modified.
Ways based on (2) or (3) scarcely provide a highly effective emission. The emission is also preferentially directed to oblique directions as in the case of (1). An increased light diffusibility checks the efficiency because of factors such as wide range scattering or absorption by light scattering elements (for example, by a white ink).
Although ways based on (4) are capable of causing light to escape from the emission face with ease, positive direction conversions are less effected. Accordingly, emission with a high efficiency is less expected. In particular, it is not advantageous that they fail to generate light rays which travel from the back face to the emission face.
To the contrary, ways based on (5) positively generate light which travels from a back face to an emission face of a light guide plate, being free from wide range light scattering.
FIG. 1a to FIG. 1c illustrate examples based on the above (5). Referring to the figures, reference number 1 indicates a light guide plate made of a transparent material such as acrylic resin, the plate having a side end face to provide an incidence end face 2. A primary light source L is disposed beside the incidence end face 2 to be supplied with light from the primary light source L. One of two major faces 3 and 4 of the light guide plate 1 provides an emission face 3. The other major face (called “back face”) is provided with a great number of recesses 5 having a cross section including slopes 5a and 5b. 
The primary light source L emits light which is introduced into the light guide plate 1 through the incidence end face 2. Upon encountering a recess, a propagation light within the light guide plate 1 (as represented by G1, G2) is inner-reflected by a slope 5a to be directed to the emission face 3. Inner-incidence angle is denoted by θ and emission beams derived from beams G1, G2 are denoted by G1′, G2′. In other words, the slope 5a, which is relatively near to the incidence end face 2 (or primary light source L) compared with the other slope 5b, provides an inner-reflection slope for direction conversion. This effect is sometimes called edge-lighting effect.
The recesses 5 are formed like dots or linear channels. As shown in FIG. 1a to FIG. 1c, formation pitch d, depth h or slope inclination φ of the recesses 5 is varied depending on distance from the incidence end face 2. Such variations prevent brightness on the emission face 3 from varying depending on distance from the incidence end face 2.
However, prior arts as shown in FIG. 1a to FIG. 1c are subject to the following problems.
1. Less light reaches a region behind the slope 5b as viewed from the incidence end face 2. Therefore, a reduced pitch d hardly rises direction conversion efficiency and the emission face 3 is apt to show an unevenness in brightness.
2. Sufficient direction control in a plane parallel to the incidence end face 2 is not effected. For instance, if beams G1 and G2 are parallel to the emission face 3 but not perpendicular to the incidence end face 2, emission beams G1′ and G2′ will be diverged to the right or left as viewed from the incidence end face 2. Actually, there is considerable light components which propagate not perpendicularly with respect to the incidence end face 2 within the light guide plate 1. Accordingly, it is difficult to provide an emission to a desirable angle or within a desirable angle range spatially (i.e. in both planes parallel and vertical to the incidence end face 2).
3. Light leaking through the back face 4 occurs easily because direction conversion for generating light directed to the emission face 3 relies upon once-occurring-reflection (at slope 5a). That is, the condition for total reflection is broken with ease at the reflection for direction conversion. For instance, if beams G1′ and G2′ are required to be directed to approximately frontal directions, inner-incidence angle θ is set at about 45 degrees. This is roughly the same as the critical angle for an interface between air and acrylic resin which is a typical material. Therefore, a considerable part of light propagating slightly downward leaks through the slope 5a. 
The present inventor proposed a light guide plate and surface light source device/LCD employing the light guide plate, which were disclosed Japanese Patent Application Tokugan-Hei 11-38977. A brief explanation of the proposed technique is as follows, being aided by FIG. 2 and FIGS. 3a, 3b. 
FIG. 2 is a plan view showing an arrangement of a surface light source device as viewed from a back side of a light guide plate arranged therein, the arrangement being disclosed in the above-mentioned patent application.
FIG. 3a is a partially enlarged perspective view of the light guide plate employed in the surface light source device shown in FIG. 2, and FIG. 3b is a partially enlarged view of one of projection-like micro-reflectors formed on a back face of the light guide plate. Note that sizes of micro-reflectors are exaggerated for the sake of explanation.
Referring to FIG. 2, a light guide plate 10 made of a transparent material. The light guide plate 10 has an end face (minor face) to provide an incidence end face 12. A back face referenced with numeral 14 is a back face provided by one of major faces. The other major faces provides an emission face (See FIG. 3a). The light guide plate 10 has right and left side end faces (minor faces) 15 and 16.
A rod-like primary light source (cold cathode lamp) L is disposed along the incidence end face 12 which is supplied with light from the light source. Both ends of the cold cathode lamp L are electrode portions EL1 and EL2 between which a light emitting portion extends with a length somewhat smaller than that of the incidence end face 12. Such a design is often employed in order to avoid the electrode portions EL1, EL2 from sticking out.
According to a basic feature of the technique disclosed in the above patent application, a great number of projections 20 are formed on the back face 14. The primary light source L emits light which is introduced into the light guide plate 10 through the incidence end face 12. An inner propagation light travels within the light guide plate 10 and is reflected generally twice when entering into one of the micro-reflectors 20, with the result that a light directed to the emission face 13 is produced. That is, the micro-reflectors function as “direction-conversion means for converting an input light into an inner output light”.
As shown in FIGS. 3a and 3b, each of the micro-reflectors 20 is configurated as to be projected from a general plane (level plane) representative of the back face 14. The illustrated micro-reflector 20 has a shape like a projection having six faces 21, 22, 23, 24, 27 and 28. Note that, in the instant description, “general level plane representative of a back face” is called “first general plane” and “general level plane representative of an emission face” is called “second general plane”.
The faces 21 and 22 provide a guiding portion to effect a smooth light input for direction-conversion. The faces 21 and 22 meet each other at a ridge portion 26. On the other hand, the faces 23 and 24 effect reflections twice for direction-conversion, producing an inner output light. The faces 23 and 24 meet each other at a ridge portion 25. The faces 27 and 28 are side walls limiting width of the micro-reflector.
An orientation of each micro-reflector is represented by an extending direction of the ridge portion 25. In the illustrated example, the ridges 25 and 26 have a straight-projection-line provided by “projecting them onto the first general plane”. Arraying of the micro-reflectors is designed so that they align to directions corresponding to light coming directions, respectively, in order to rise input efficiency and direction-conversion efficiency.
A great part of input light represented by beams H1, H2 is incident to the incidence end face 12 in a direction approximately perpendicular to the incidence end face 12. However, light that is actually inputted into the projections is not precisely parallel to the first general plane but progresses somewhat downward (as to approach the back face 14).
Light that progresses precisely parallel to the first general plane or approaches the emission face 13 advances deep without being inputted to projections 20. Consequently, the projections 20 do not obstruct light advancing and give no region little light reaches, thereby effecting contrary to recesses (See shown FIG. 1).
Viewing from the standpoint of the beams H1 and H2, the reflection faces 23 and 24 of the conversion output portion configurate a valley getting tapered forward. The ridge 25 corresponds to a bottom of the valley. The valley gets narrower and shallower according to distance from the guide portion. Therefore, a great part of light H1 and H2 entering the valley via the guide portion is inner-reflected by one of the reflection faces 23 and 24, and then inner-reflected again by the other faces 24 or 23.
As a result, a light propagation direction is converted twice three-dimensionally to produce inner output light J1, J2 directed to the emission face 13. The inner output light J1, J2 produce in such a way is emitted from the emission face 13 and used for illuminating an object such as LCD panel. Various variations of arrangement and orientation of the micro-reflectors 20 are allowed. The example shown in FIG. 2 is subject to the following rules.
1. Formation density (covering rate) tends to increase according to distance from the incidence end face 12. This prevents brightness on an emission face from varying depending on distance from the incidence end face 12.
2. Micro-reflectors 20 are arranged in corner areas A, B near to the electrode portions EL1, EL2 at a specially large density. This prevents, together with orientation of the following item 4, prevents dark areas corresponding to the areas A, B from emerging on the emission face.
3. Micro-reflectors 20 are orientated so as to be approximately vertical to the incidence end face 12 almost over the back face 14, with their guide portions being directed to the incidence end face 12. In other words, each micro-reflector 20 is orientated so that its conversion output portion has a ridge 25 which extends approximately at the right angle with respect to the incidence end face 12.
4. In the corner areas A, B, micro-reflectors 20 are much obliquely orientated with respect to incidence end face 12, with guide portions being directed to the light emitting portion of the cold cathode lamp L. This causes these micro-reflectors 20 to be orientated as to be corresponding to light coming directions, thereby rising direction conversion efficiency.
5. In both side edge portions 15, 16 except the corner areas A and B, micro-reflectors 20 are orientated so as to be inclined at small angles with respect to the incidence end face 12, with guide portions being directed to the light emitting portion of the cold cathode tube L. This causes these micro-reflectors 20 to be orientated corresponding to light coming directions, as the above item 4, thereby rising direction conversion efficiency. If conversion output portions (directions of inner reflection faces 23 and 24) of micro-reflectors 20 located in a certain range from both side end faces 15 and 16 are designed the so that an inner output light is inclined toward a center portion of the light guide plate 10, an emission with converging property is produced.
6. Micro-reflector arrangement does not have a striking regularity such that many micro-reflectors 20 align along a straight line. This makes the micro-reflectors 20 more inconspicuous. And besides, if incorporated in a liquid crystal display, the micro-reflectors can avoid from bringing Moire fringes which would be caused by an overlapping relation with a matrix-like electrode arrangement.
It is possible to heighten the performance of a light guide plate and surface light source device/LCD employing the light guide plate, which were disclosed in the above propose, by adding contrivances as above.
However, the above-proposed technique remains a problem unsolved. That is, the proposed technique, if applied, a fine unevenness in brightness appears on the emission face of the light guide plate 10 corresponding to the size and arrangement pitch of the micro-reflectors 20. This gives a viewer a non-smooth visual feeling (a feeling of glaring).
This problem is supposed to arise due to a fact that a roughly almost of the inner output light of the micro-reflectors 20 escapes and is emitted from the emission face at the first chance with ease, as mentioned with referring to FIGS. 3a and 3b. In the instant description, such an escaping (light) at the first chance is called “direct escaping (light)”.
Needless to say, such direct escaping processes occur generally corresponding to positions of the micro-reflectors 20. On the other hand, an efficient emission can not be expected in a blank region (a flat region on the flat back face 14) without micro-reflectors 20 among the micro-reflectors. As a result, a fine unevenness in brightness appears on the emission face.
In the instant description, the term “indirect escaping” or “indirect escaping light” means a phenomena or escaping light which occurs or generates at second or later chances after being inner-reflected by the emission face. Simply saying, if the direct escaping light is produced to much as compared with the indirect escaping light, a fine unevenness in brightness will appear.
This problem will be relaxed to a degree by arraying the micro-reflectors 20 at a high density. However, arraying density is subject to a practical limit.